An instrument embodying the invention is of a known general type including a seat having two seat sections which support the right and left sides respectively of a person's body and are movable to change the positions of the two sides of the body relative to one another. Such devices have been used in the past for attempting to correct the posture of a person who may normally tend to sit in a position in which one side of the body is lower than the other or at a different inclination. The general procedure has been to first allow a patient to sit on the sectionally formed seat in the position which seems normal to him, following which an operator adjusts the two sections of the seat relative to one another until the proper orientation of the body is attained. The seat and patient are kept in this adjusted position for a short period of time on each treatment, in order to acquaint the patient with the proper posture and hopefully encourage maintenance of that posture after the treatment has been completed. In prior instruments of this type, one of the seat sections has been power actuable upwardly and downwardly relative to the other, and each of the seat sections has in addition been mounted for individual powered tilting movement in two different directions, to raise or lower the forward end of the seat section or raise or lower its outer edge relative to its inner edge. These various powered movements of the two seat sections have been attained by a motor having a series of sprocket type transmission assemblies adapted to be controllably and selectively connected to the motor to produce any desired type of movement under the control of a series of switches manually actuated by an operator.